Well, Twitter has flipped me the bird.
When I sent in a support request, asking what this was about, a few folks pointed me to a wiki talking about rate limiting - i.e. preventing certain actions for a period of time, like queries to the API.
However, after 4 days, things didn't get better, so I opened a support case in as many ways as I could. Here was the response I got back:
The follow limit is different for every profile, based on that profile's activity. If you've hit the follow limit for your profile, you can un-follow some people in order to follow others. You can stop following some people to follow others, however, you wont be able to follow more people than you're following currently.
Twitter has Pulver-ized me - Jeff Pulver encountered the 5K ceiling with Facebook, and had to open a second account.
For those of you who have followed back as many people as you could, be glad you did - that may be all you ever get.
For those folks who view following back as important, I'm sorry to say that Twitter is now preventing that, at least for me.
In the meantime, I'm going to go over to Plurk and play around there some. You're welcome to join me off Twitter at the following: